Manufacture of luminescent materials



Patented July 2, 1940 MANUFACTURE OF LUMINESCENT MATERIALS Herman C.Froelich, Cleveland, Ohio, assignor to The Harshaw Chemical Company,Elyria, Ohio, a corporation of Ohio No Drawing. Application January 20,1940,

Serial No. 314,797

16 Claims.

This invention relates to luminescent materials, particularlyluminescent silicates, and has for its principal object the productionof luminescent silicates of improved afterglow.

Strong afterglow is desirable for bridging over variable or intermittentstates of excitation so as to give an even illumination, reference beinghad to use in electric discharge tubes.

This application is a continuation-in-part of my co-pending applicationSerial No. 227,987, filed September 1, 1938.

Broadly stated, my invention comprises the discovery that luminescentsilicates of improved afterglow are provided by the incorporationtherein of a suitable small quantity of arsenic. I have found that mydiscovery applies to luminescent silicates generally, and is ofoutstanding value' in the cases of zinc silicates, cadmium silicates,beryllium silicates, zinc-beryllium silicates andzinc-magnesium-beryllium silicates.

In accordance with the invention, I prepare luminescent silicates ofimproved afterglow by firing together, in an oxidizing atmosphere orunder equivalent oxidizing conditions, the following:

(1) Silica, preferably prepared as herein described,

(2) A suitable metal oxide or mixture of metal .oxides capable offorming luminescent silicates or compounds of such metals capable ofyielding the oxides upon calcination, preferably one of i the following,which I have found to give superior results:

(a) Zinc oxide or zinc oxide yielding material such as the nitrate orcarbonate. 1

(b) Cadmium oxide or cadmium oxide yielding material such as thecarbonate or the nitrate. Magnesium may be substituted for a portion ofthe cadmium.

(c) Zinc oxide mixed with beryllium oxide or the oxide of either zinc orberyllium mixed with the silicate of the other. Suitable oxide yieldingcompounds such as the nitrate or carbonate may be substituted for theoxide. A portion of the beryllium may be replaced by magnesium as ticeof the invention, other ingredients may be used for special purposes aswill be obvious to persons skilled in the art.

Silica derived from suitably prepared ammonium silicofluoride orsuitably prepared 5 fluosilicic acid is unusually well suited to theproduction of these luminescent silicate materials.

I prefer to use, as a starting material for preparation of such silica,sodium-silicofluoride 0 which usually will contain small amounts ofarsenic, iron, lead and sometimes other heavy metals. The iron, andother impurities as well, should for duplicability of results be almostentirely removed, while the arsenic should be pres- 15 ent in criticalquantity if a strong afterglow is desired. If it should be desired toreduce the afterglow to a minimum, the arsenic content should be verylow (e. g. less than .0001%). I prefer to remove the arsenic and add thecritical 20 amount at the proper stage.

In accordance with the preferred practice of the invention, sodiumsilicofiuoride is reacted with an excess of concentrated sulfuric acidin the presence of sand or like siliceous material 25 at a temperatureof the order of C. in a suitable still. The sulfuric acid used is ofsuch strength and used in such quantity as to bind the water in thereaction chamber and prevent the formation of hydrofiuosilicic acid. Thesand 30 reacts with the hydrofluoric acid generated so that the onlyvolatile product is silicon tetrafluoride. Non-volatile impurities areleft in the still. Volatile impurities can be removed later in thecalcination step.

The silicon tetrafiuoride is now run into aqueous hydrofluoric acid,which, having been made from distilled anhydrous acid and distilledwater, carries only volatile impurities, removable in the calcinationstep. This results in the for- 40 mation of hydrofluosilicic acid ofhigh purity, which is then converted by ammonia into ammonium fluoride,silica and water. The ammonia may be gaseous or ammonium hydroxide madeby absorbing ammonia gas in distilled 45 Under the microscope, thepowder, dispersed in quinoline, appears opaque, whereas a silica in theform of a ground, dried gel appears transparent. Previously knownmethods of making silica do not, so far as I am aware, give, at the sametime, high purity and a physical state most desirable for the productionof luminescent materials.

As indicated above, this precipitate may contain volatile impuritieswhich, I have found may,

according to choice, be expelled by heating to incipient red heat (about500 C.) or be left in the material to be expelled on the finalcalcination in the production of the luminescent materials. It will benoted that the final solution from which the silica is precipitatedcontains only NI-I4F in solution. Accordingly, no nonvolatile impurityis present because of incomplete separation of the products.

Other silicofiuorides which yield SiF4 may be used as source materialsinstead of NazSilFc. Ordinary commercial (NH4)2SlFs cannot be used inthe final reaction unless it is first specially treated to removenon-volatile impurities. This may be done by a process as follows:

(a) A saturated aqueous solution of (NH4) ZSiFS is acidified with HzSiFssolution sufficient to produce 5% free acid. The amount is not critical.

7 (b) The solution is treated with H2S, filtered and excess HzS blownout with air.

(0) The filtrate is nearly neutralized with NHlOH, treated with H28,filtered and the excess H2S blown out with air.

(d) The filtrate is treated with an oxidizing agent such as H202, cooledbelow 10 C. and treated with ammonium nitroso phenyl hydroxylamine(Cupferron) and filtered. The resulting (NH4) zSiFs can be used in thefinal reaction and gives a silica of the required purity and in thedesirable physical state.

Fluosilicic acid of suitable purity is suitable as a source of silica.Silica can be precipitated therefrom by the use of ammonia.

Following are specific examples of silica preparation:

' EXAMPLE I 760 grams of Na2SiFs, 240 grams silica sand, and 800 gramsof concentrated H2804 were brought together in a still at a temperatureof 150 C. The SiF4 evolved was led into a solution of 20% aqueous HF,cooled with tap water while HF was also led into the solution in suchquantity that neither free HF norfree Si02 could be titrated. A slightexcess of concentrated NH4OH was added and-the precipitate of silicacomposition separated from the solution. On analysis, the calcinedsilica showed less than 003% of nonvolatile impurities (mainly Mg andFe, spectroscopically identified).

EXAMPLE 11 Same as Example I except that the precipitate was heated to500 C. for two hours.

EXAMPLE III Example I was varied by the addition to the reaction mixturejust before the final precipitation of 0.3 gram of AS203 in watersolution for each 1000 grams of SiOz produced.

The quantity of arsenic to be employed for best results appears to berelated to the silicon content of the final zinc silicate compositionand it is convenient although not necessary to incorporate the properamount of arsenic in the silica after which standard practice may befollowed in the production of the luminescent silicates, using thespecial arsenic-containing silica where silica of usual derivation hasbeen used heretofore.

The proportion range for arsenic, for best results, should be of theorder of 0.003% to 0.03%'

of the S102 content, arsenic being for the calculation treated aselemental, and SiOz being treated as such even though probably beingentirely in combined form in the final product. Fair results may be hadoutside this range, for example in the range .001% to .05%, arsenicbased upon SiOz as before. This material exhibits the property of beingexcitable by sunlight. The activator, preferably Mn(NO3)2 or MnClz, maybe employed in usual, concentrations such as from 3 to 10 per cent ofthe S102. Proportions of other constituents may vary as indicated in theexamples following. If instead of calculating the arsenic content asparts of elemental arsenic per 100 parts SiO2, which gives a preferredrange of 0.003% to 0.03%, the arsenic content is calculated as parts ofAS203 per 100 parts SiO2, the result will be for the same preferredrange approximately 0.004% to 0.04%. Again, if the calculation, in thecase of a zinc silicate is based upon parts of AS203 per 100 parts zincsilicate, the same preferred range will be expressed as 0.001% to 0.01%.

EXAMPLE IV I may, for example, grind parts by weight (dry basis) ofsilica containing from 003% to 03% arsenic, either dried at atemperature of the order of 500 C. or preferably uncalcined, with from190 to 380 parts of Zn(NO3)2 and from 2.5 to 5 parts of Mn(No3)2 andsufficient water to produce a thin slurry. If the silica compositionisused as a wet cake such as obtained after filtering, it will yieldpure silica upon subsequent calcination since it was produced from rawmaterials which contained no non-volatile impurities. The thin slurrymay be evaporated to dryness, fired to about 600 C. to expel all watervapor and nitric gases, reground in a ball milland again fired,preferably at 800 to 900 C. until all volatile fluorides are expelledand then fired to about 1200 C. for one-half hour or longer. Theresulting zinc silicate composition shows strong green fluorescenceafter suitable excitation and a surprisingly brilliant afterglow.

EXAMPLE V Cadmium silicate EXAMPLE VI Zinc beryllium silicate 324 gramszinc oxide, grams beryllium oxide, 2'76 grams silica (dry basis) in formof a wet filter cake, 19.6 grams manganese chloride, 30 mg. of A5203 inaqueous solution and sufficient water to form a slurry are ball milledtogether until homogeneous. Drying, etc. is carried out as in Example V.The final calcina- 2,206,280 tion temperature may be 1100 to 1300 C.(preterably 1220 (7.).

EXAMPLE VII Zin'c beryllium ma nesium silicate crystals, 25 mg. of AS203in aqueous solution and sufficient water to form a slurry are treated asbefore and finally calcined at about 1090 C.

(Limits: 1000-1150 C.)

Having thus described claim is:

1. A luminescent silicate material having incorporated therein aquantity of an oxide of arsenic suflicient to produce a content of from.001% to .05% of arsenic based upon the weight of SiOz, whereby toimpart to said material an improved afterglow.

2. A luminescent silicate material having incorporated thereinfrom .001%to .05% of arsenic based upon the weight of $102.

3. A luminescent silicate material having incorporated therein from 003%to 03% of arsenic based upon the weight of S102.

4. A manganese activated, luminescent zincmy invention, what I berylliumsilicate having incorporated therein from .001% to .05% arsenic basedupon the weight of SiOz.

5. A manganese activated, luminescent cadmium silicate havingincorporated therein from .001% to .05% of arsenic based upon the weightof SiO2.

6. A manganese activated, luminescent zincberyllium silicate havingincorporated therein from 003% to 03% arsenic based upon the weight ofS102.

'7. A manganese activated, luminescent cadmium silicate havingincorporated therein from a 003% to 03% of arsenic based upon the weight8. A new composition of matter comprising a luminescent silicate beingthe product of a process comprising precipitating silica in anon-colloidal state from a solution of a substance of the groupconsisting of ammonium silicofluoride and fluosilicic acid, allreactants being substantially free from non-volatile impurities, andcalcining the resulting silica with a sutable activating agent and acompound capable of combining therewith to form a luminescent silicatecomposition.

9. As a new composition or matter, a luminescent zinc silicatecomposition, being the product of a process comprising precipitatingsilica in a non-colloidal state from a solution of a substance of thegroup consisting of ammonium silicofiuorlde and fiuosilicic acid, allreactants being substantially free from non-volatile impurities, andcalcining the resulting silica with a suitable activating agent and azinc compound capable of combining therewith to form a luminescent zincsilicate composition.

10. Process of making a silica composition suit able for production of aluminescent material comprising coprecipitating from a solutionsubstantially free from non-volatile impurities a composition containingoxides of silica and'arsenic, the latter amounting to from .004% to 04%of the former.

11. A silica composition, suitable for production of a luminescentmaterial, comprising highly pure silica containing from 004% to 04% ofoxide of arsenic. l

' 12. A luminescent zinc silicate composition of high purity, the samecontaining from 004% to 04% of oxide of arsenic, calculated on the S102content and being excitable by sunlight.

13. A luminescent zinc silicate composition containing from .001% to.01% oxide of arsenic, the percentage of arsenic being calculated on thebasis of the weight of the zinc silicate.

14. A manganese activated zinc silicate composition containing from.001% to 01% oxide of arsenic, the percentage of arsenic beingcalculated on the basis of the weight of the zinc sili- 15. A process ofproducing a luminescent zinc silicate composition comprising intimatelyadmixing 60 parts by weight of highly purified, finely divided silica,in a physical state to appear opaque when dispersed in quinoline, 190 to380 parts of solution a silica, containing NH4F as an impurity,

and, in such physical state that if a sample is dispersed in quinolineit will appear opaque, admixing the same, uncalcined, with a zinccompound and an activating material in proportions suitable for formingluminescent zinc silicate and calcining the resulting mixture.

, HERMAN C. FROELICH.

